Double Star of the Month:
Xi Ursae Majoris
AKA: Ksi Ursae Majoris; Alula Australis; STF 1523
Position: 11 hr 18 min 11 sec +31 degrees 31 min 45 sec
Due south at 22:44 (BST) on 15 April 2022
Image credit: Jeremy Perez (http://www.perezmedia.net/beltofvenus)
Used with permission
Last month I mentioned that there are only seven double stars visible from Havering where both stars are brighter than magnitude 4. If I take magnitude 5 as the minimum brightness, the number increases to 20, including Gamma Arietis (Double Star of the Month for November 2019), Albireo (August 2020), Beta Monocerotis (January 2021), k Puppis (February 2021), Epsilon Lyrae (July 2021), and Alya (August 2021). This double star is also on that list. Xi Ursae Majoris (Ksi UMa as far as Simbad is concerned) shows that the constellation of Ursa Major extends far beyond the Plough and this double is almost in Leo Minor. Xi UMa is close to Earth being only 28.9 light years distant. Its odd name Alula comes from the Arabic for “the first leap”. Alula Australis (magnitude 3.8) is the southern star of the pair; the more northern Alula Borealis (Nu Ursae Majoris) is slightly brighter (magnitude 3.5). The pair are easily found in a barren field roughly halfway between Dubhe (magnitude 2.0) and Denebola in Leo (magnitude 2.3).
The duplicity of Xi UMa was discovered by William Herschel on 2 May 1780 and it was the first binary to have its orbit calculated, by the French astronomer Félix Savary, in 1828. The main star is magnitude 4.3 and the secondary is 4.8. It is a difficult star to split in a small telescope as the separation is only 2.3 arcseconds and you will need to use a high magnification; at least 150x and possibly 200x. However the situation has improved since the 1990s when the separation was only 1.6 arcseconds and it will have widened to 2.9 arcseconds by 2030. The two stars are close together, only 2.5 AU apart, and the period is 59.9 years. They are similar main sequence stars, the main star being spectral type F8.5 and the secondary is G2. They are both spectroscopic binaries. Given their spectral types, one would expect them to be yellowish-white but they are often seen as white, for example, by Herschel, perhaps with tints (e.g. Thomas Webb, 1881). Alula Borealis is also a double star, but the secondary is faint. The main star is magnitude 3.6 and the secondary is 10.1; the separation is a reasonable 7 arcseconds. There is a third member which is 287 arcseconds away, perpendicular to the other two stars, and magnitude 8.9. As it is spectral type K3, the main star is usually seen as deep yellow or orange; it is similar to the main star of Albireo.